Fibers with non-circular cross-sections are more desirable since they possess some unique properties compared to those fibers with circular cross-sections. First, they have special optical effect, particularly for those fibers with triangular cross-sections. The triangular cross-section can act as a small prism to split the incoming light and then recombine them, thus producing a desirable lustrous effect. Second, the non-circular structure offers greater surface area. The greater surface area can enhance area coverage, reduce the transparency of textile fabric, and improve pilling resistance. Third, the special shape of non-circular cross-sections could enhance the cohesion among fibers, and provide the bulkiness for better air permeability to fibers. Finally, fibers with non-circular cross-sections also have superior snagging resistance. Therefore, fibers with non-circular cross-sections have been widely used in weaving, needling, knitting, and carpet industries. Polyimide (PI) fibers, as one of the high-performance organic fibers, possess superior thermal resistance to both high and low temperatures, exceptional radiation resistance and chemical solvent resistance, good electrical insulation properties, and excellent mechanical properties. Therefore, PI fibers have become increasingly important in a variety of technological applications, such as applications requiring operating at high or low temperatures, under harsh chemical environments, and in high-performance composites. Having combined characteristics of PI fibers with circular cross-sections, PI fibers with non-circular cross-sections will be very promising materials for special textile and filtration applications.
The most common method for making fibers with non-circular sections is to use non-circular spinneret orifices. Existing PI fibers with non-circular sections are prepared by this traditional method. For example, the commercially available product, P84, which is a PI fiber with a trilobal cross-section, is prepared through trilobal spinneret orifices. P84 has been widely used as a high temperature resistant material for filtration. However, since the size of the spinneret orifices had to be very small, it is difficult to machining spinneret plates having circular orifices, and it is even more difficult and costly to machining spinnerets plates having non-circular orifices. Therefore, there remains a need to develop methods for making PI fibers with non-circular cross-sections without using non-circular shaped spinneret orifices, to greatly simplify the manufacturing process, improve the production efficiency and lower the production cost.